The present invention relates to an exhaust system for use with an internal combustion engine, in particular to a two-stroke internal combustion engine using a normally gaseous hydrocarbon as fuel. The present invention further relates to a two-stroke engine incorporating the exhaust system. Finally, the present invention relates to a method for operating a two-stroke internal combustion engine.
Two-stroke (alternatively referred to as two-cycle) engines have been known for many years and have been applied in a range of applications. One class of two-stroke engines is the class of engines operating on a normally gaseous hydrocarbon, most commonly natural gas. Such engines are generally large, slow running engines of a stationary design and find application in the driving of rotating equipment, such as compressors. One example of commercially available engines are the Ajax series of engines manufactured and sold by the Cooper Energy Services division of Cooper Cameron Corporation. The Ajax engines are two-stroke engines having from one to four cylinders. When used to drive a compressor, the Ajax engines are commonly employed in a configuration in which the cylinders of a reciprocating compressor are driven from the same crankshaft as the cylinders of the engine.
Engines in the aforementioned class generally operate at low speeds, that is speeds of the order of from several hundred to a thousand revolutions per minute. The engines are generally operated in a constant speed mode, in which a substantially constant speed is maintained under a variety of engine loads. As the power demand placed on the engine is increased, the combustion efficiency and performance of the engine decreases. In particular, the levels of NOx in the exhaust gas increases. At a certain level of power demand, the concentration of NOx in the exhaust gas will exceed the maximum concentrations permitted by the environmental regulations. At this point, the engine can be considered to have reached its maximum acceptable power output. Currently, there is increasing pressure to reduce the level of NOx emissions from this class of engines. Thus, at present, it is possible to operate such engines under conditions producing NOx at the rate of 2 g/HP-hr. However, in many regions the relevant regulations already require that NOx emissions be limited to just 1 g/HP-hr. Still further regions are moving to adopt similar regulations reducing the permitted levels of NOx emissions. Accordingly, there is a need for a way to reduce NOx emissions from engines in this class.
One method of reducing the amount of NOx emitted by such engines is to reduce the maximum power rating of the engine and to operate the engine at a lower level of power output. While this will allow the engine to operate within the lower limits placed on permissible NOx emissions, this method of operation also limits the useful power obtainable from the engine and, hence, its range of useful applications. An alternative approach is to employ a catalytic converter in the exhaust system of the engine. The catalytic converter removes the NOx from the exhaust gases before they are emitted to the atmosphere. However, it will be appreciated that the addition of a catalytic converter increases the expense of the engine. In addition, the use of a catalytic converter increases the maintenance required to be performed on the engine, increasing the down time of the engine and, again, increasing costs.
Accordingly, there is a need for a solution to the problem of achieving lower NOx emissions, while allowing the engine to maintain its existing power rating. The solution should also be available at minimum cost and require little or no additional maintenance over and above the existing maintenance schedules.
Surprisingly, it has been found that the efficiency of the combustion of the aforementioned two-cycle engines can be significantly increased by the use of an exhaust system tuned to the range of operating speeds of the engine. In particular, this improved efficiency allows the engine to operate with significantly lower NOx emissions than using a standard exhaust system, while allowing the power output of the engine at the operating speed to remain at the previously achieved levels, or even increased.
Accordingly, in a first aspect, the present invention provides an exhaust system for an internal combustion engine, the engine being a two-cycle engine combusting a normally gaseous hydrocarbon as fuel, the engine comprising a cylinder having an exhaust port in communication with an exhaust manifold, the exhaust system comprising:
an exhaust gas inlet, the inlet being connectable to the exhaust manifold of the internal combustion engine, the exhaust system having a direction of flow being the direction of flow of exhaust gases moving through the exhaust system when the system is connected to the exhaust manifold of the internal combustion engine and the engine is operating;
an exhaust gas outlet spaced from the exhaust gas inlet in the direction of flow, the outlet being connectable to a muffler;
a first tubular portion disposed between the exhaust gas inlet and outlet, the first tubular portion diverging in the direction of flow;
a second tubular portion disposed between the exhaust gas inlet and outlet and spaced from the first tubular portion in the direction of flow, the second tubular portion converging in the direction of flow;
wherein, during operation of the engine to which the exhaust system is connected, a first, negative pressure pulse is generated in a direction opposite to the direction of flow to enhance the removal of exhaust gases from the cylinder through the exhaust port and a second, positive pressure pulse is thereafter generated in the direction opposite to the direction of flow to prevent the flow of freshly charged gas from the cylinder.
While exhaust systems tuned to the operating speed of a two-stroke engine are known in the art, their application has been limited to two-stroke engines of much smaller capacity and operating at speeds far greater than those of the class of engines addressed by the present invention. To date, such a tuned exhaust has not been applied to large capacity two-stroke engines operating on a normally gaseous hydrocarbon fuel and operating at speeds at or below about 1000 rpm. Further, it is most unexpected that such a tuned exhaust can be employed to reduce the NOx emissions of the engine without sacrificing power output from the engine. It is particularly surprising that the low levels of NOx emissions can be achieved with the exhaust system of the present invention while achieving increased power output from the engine at a given speed of operation.
In a second aspect, the present invention provides a two-stroke internal combustion engine comprising a cylinder having an inlet port and an exhaust port, the engine further comprising an exhaust system connected to an exhaust manifold in communication with the exhaust port, the exhaust system comprising:
an exhaust gas inlet connected to the exhaust manifold of the internal combustion engine, the exhaust system having a direction of flow being the direction of flow of exhaust gases moving through the exhaust system when the engine is operating;
an exhaust gas outlet spaced from the exhaust gas inlet in the direction of flow;
a muffler connected to the exhaust gas outlet;
a first tubular portion disposed between the exhaust gas inlet and outlet, the first tubular portion diverging in the direction of flow;
a second tubular portion disposed between the exhaust gas inlet and outlet and spaced from the first tubular portion in the direction of flow, the second tubular portion converging in the direction of flow;
wherein, during operation of the engine, a first, negative pressure pulse is generated in a direction opposite to the direction of flow to enhance the removal of exhaust gases from the cylinder through the exhaust port and a second, positive pressure pulse is thereafter generated in the direction opposite to the direction of flow to prevent the flow of freshly charged gas from the cylinder.
In a further aspect, the present invention provides a method of operating a two-stroke internal combustion engine, the method comprising:
supplying a normally gaseous hydrocarbon fuel and an oxygen-containing gas to a cylinder of the engine to provide a combustible mixture within the cylinder;
combusting the combustible mixture to produce an exhaust gas;
allowing the exhaust gas to leave the cylinder through an exhaust gas outlet port;
passing the exhaust gas through a first exhaust gas conduit, the first conduit diverging in the direction of flow of the exhaust gas so as to allow the exhaust gas to expand; and
passing the exhaust gas through a second exhaust gas conduit, the second conduit converging in the direction of flow of the exhaust gas so as to compress the exhaust gas;
whereby a first, negative pressure pulse is propagated through the exhaust gas in the opposite direction to the direction of flow to reach the exhaust port and thereby assist with removing exhaust gas from the cylinder; and
a second, positive pressure pulse is propagated through the exhaust gas in the opposite direction to the direction of flow to reach the exhaust port and thereby prevent freshly charged gas from leaving the cylinder through the exhaust port.
Specific embodiments of the apparatus and method of the present invention will now be described in detail having reference to the accompanying drawings. The detailed description of these embodiments and the referenced drawings are by way of example only and are not intended to limit the scope of the present invention.